const fs = require('fs')
fs.writeFile('1.txt','123456',(err)=>{
    if (err) throw err
    console.log('111')
})
fs.writeFile('1.txt','789',(err)=>{
    if (err) throw err
    console.log('111')
    fs.readFile('./1.txt',(err,data)=>{
        if(err) throw err;
        console.log(data.toString())
    })
})
fs.appendFile('1.txt','999')